These are likely sea bass, and I catch a few of them to prove that the transducer is telling the truth. I mark the spot by hitting the MOB button on the chartplotter, then drop a plastic milk jug rigged with a line and heavy sinker, so that I also have a visual target to work from as I make more passes over the tank.
TRANSOM MOUNT
Then it's time to pull the plug. I switch over to the transom mount. Transom mounts are loved for their easy installation. Anyone with a basic ability to use a drill and screwdriver can do it (use plenty of 3M 5200 sealant, since the screw holes are below the waterline) and there's no danger of sinking your boat if you botch the job. Like the through-hull, the P-66 transom mount I'm testing has a dual-frequency, 50/200-kHz crystal inside a plastic casing. It also has a hydrodynamic nose cone to reduce turbulence, and can be used with a wide variety of different units from different manufacturers.
CREDIT TKA dual-frequency 1kW transducer; 19 degrees at 50kHz and 6 degrees at 200kHz.
The tank now appears to be a mere mass – a pile of something, made up of red and green blocky images with a few yellow blobs nearby. If the image with the through-hull was a perfect 10, the transducer mount scores an imperfect eight.
Time to switch wires again. I plug in the P-79 shoot-through, which is also a dual 50/200 frequency unit. Unlike the other transducers, this one is designed to mount inside the hull, and I bonded it to the Albemarle then filled its base with mineral oil before twisting the offset ring to account for the hull's 21-degrees of deadrise.
Obviously, there are some huge advantages to mounting your transducer inside the boat: hydrodynamics, ease of mounting, and safety all get a boost. But you can't use this form of transducer if your boat has a cored bottom. It must be solid glass, and glass bottoms laid up carelessly, with air pockets and gaps, won't work either. Lucky for me, the Albemarle has a solid glass bottom that's been built properly.
Just one problem: as I make the next pass the tank is smaller and flattened out into a fuzzy lump. The colors become even less distinguished, and what were separate targets indicating fish have become part of the whole mass. It's a score of five or six, at best. The difference between these pictures is as dramatic as the difference between first-generation digital cameras and today's tack-sharp, 10 mega-pixel professional models.
Okay: so now we know how performance relates to transducer type in shallow water, when looking at hard structure and fish. Plus, there are plenty of sea bass in the cooler at this point. But what will happen when I go deep? Will I experience the same results at 650 feet as I did at 65 feet? And what about when the boat is off the edge of the continental shelf, in water that's more than 1,000 feet deep? To find out, I point the bow for Baltimore Canyon, 65 miles off the coast of the Delmarva Peninsula.
DEEP WATER TEST
Two hours later, I switch the unit over to 50-kHz (remember, low wavelengths are for deep penetration), plug in the through-hull and take a few passes over the drop. In 723 feet, I see two red dots near the bottom, and send a few squid spiraling into the depths on deep-drop rigs weighted with three pound sash weights. It's not too long before a crewmate cranks up a 10-pound tilefish. Yup, those marks were real. Heading east, the unit holds the bottom with a solid reading to about 1,800 feet. After that it's a broken reading down to 2,010 feet, when the screen finally goes blank.
The transom mount does pretty well, but can't match the through-hull's performance. It holds bottom down to between 900 and 1,000 feet, but is faint and broken beyond the 800-foot mark. The slower I go, the better the performance, and the transom-mount works best when I shift into neutral and drift. This reinforces one of the things you've probably heard about transom mounts: You have to get the placement just right. Turbulence effects performance whenever the boat's moving. My placement is good, and I get a solid reading in water under 800-feet even when I'm running at cruising speed of 25 knots, but turbulence still has some effect.
